There has been widely used as a satellite broadcast receiving antenna a parabolic antenna including a parabolic reflecting mirror and a primary radiator. As shown in FIG. 8, a primary radiator for a parabolic antenna used includes a radiator body 103 having a waveguide 101 and a horn part 102 provided at one end of the waveguide 101, and a waterproof cover 104 covering an open end 102a of the horn part 102 for preventing rainwater from entering the radiator body. In the example in FIG. 8, the waveguide 101 is a circular waveguide, and an inner surface of the horn part 102 is a conical tapered surface 102b having a cross section gradually increasing toward the open end. The waterproof cover 104 is formed into a cap shape, an open end thereof is a fitting portion 104a, and the fitting portion is fitted in a liquid-tight manner to an outer periphery of an end of the horn part 102 via an O-ring 105. The radiator body 103 and the waterproof cover 104 constitute a primary radiator 106.
In this primary radiator, the horn part 102 is placed in the vicinity of the focus position of a parabolic reflecting mirror. Radio waves from a broadcast satellite, collected in the horn part 102 by the reflecting mirror, are converged by the horn part 102 and transmitted through the waveguide 101 to an unshown down converter, and signals output from the down converter are transmitted through a coaxial cable to a tuner. The down converter converts signals in a 12 GHz band received through the primary radiator 106 to signals in a 1 GHz band in order to reduce transmission loss that occurs in the coaxial cable. Such a primary radiator is disclosed as a related art in Japanese Patent Application Laid-Open No. 8-167810.
The waterproof cover 104 is generally made of resin, and has a dielectric constant of about 2 to 4. If such a waterproof cover is attached to the open end of the horn part 102 of the primary radiator 106, multiple reflection of radio waves occurs in the primary radiator to increase reflection loss.
In order to prevent multiple reflection and reduce reflection loss, in the conventional primary radiator, a distance L from an inner surface of the waterproof cover 104 to the open end 102a of the horn part 102 measured on a central axis of the waveguide 101 is set to about one-half of a wavelength λ of a radio wave to be received as shown in FIG. 8. When the radio wave to be received is 12 GHz, the distance L is about 12 mm.
When the distance L between the inner surface of the waterproof cover 104 and the open end of the horn part 102 is thus adjusted to prevent multiple reflection, it is necessary to set the distance L to be long, which causes the waterproof cover 104 to excessively project forward from the horn part 102 as shown, and snow may accumulate on the waterproof cover 104 to cause poor reception.
Thus, as disclosed in Japanese Patent Application Laid-Open No. 8-167810 and U.S. Pat. No. 6,501,432, a primary radiator has been proposed in which a projection is integrally provided on an inner surface of a waterproof cover 104 during molding of the waterproof cover 104 to prevent multiple reflection and reduce reflection loss. If the projection having an appropriate thickness is provided on the inner surface of the waterproof cover, radio waves reflected on the waterproof cover can be cancelled out by the projection, thus preventing multiple reflection and reducing reflection loss even if a distance between the waterproof cover and an open end of a horn part is short.
However, by such a method of integrally forming the projection on the inner surface of the waterproof cover, an outer surface of the waterproof cover may be dented at the projection during injection molding of the waterproof cover, and snow may accumulate on the dent to cause poor reception.
Forming the projection on the inner surface of the waterproof cover causes an intricate shape of the waterproof cover and thus an intricate structure of a die used for molding the waterproof cover, thus increasing the cost of the waterproof cover.
Further, integrally forming the projection on the inner surface of the waterproof cover causes a dielectric constant of the projection to be as high as that of the waterproof cover, thus increasing dielectric loss that occurs in the projection.
Then, as disclosed in U.S. Pat. No. 6,501,432, a primary radiator has been proposed in which a reflection preventing member constituted by a dielectric substance having a lower dielectric constant than a waterproof cover is placed in a horn to prevent multiple reflection and reduce reflection loss.
However, such a configuration requires the refection preventing member formed separately from the waterproof cover and incorporated into the radiator body, thus increasing the number of parts, causing an intricate structure, and inevitably increasing the cost.